docs: def items_to_extract(self, offset=0, length=None):
          """
          Return an iterable of specific items to extract.
          As a side-effect, set self.items_to_extract_length.
  
          :param offset: where to start extracting
          :param length: how many to extract
          :return: An iterable of the specific
          """
          endoffset = length and offset + length
          qs = self.origin_data()[offset:endoffset]
          self.items_to_extract_length = qs.count()
          return qs